Is there a January Effect If so What might explain the difference in the month effect between the first part of the sample and the second part of the sample?

Is there a weekday Effect? What might explain the weekday effect?

Monthly returns 1998-current
S&P 500		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.00160206	0.011052928	0.024942349	0.002709431	-0.00599	0.01531016	-0.00168112	-0.0121413	0.02230626	0.02050214	0.008579	0.003531683
Russell 2000		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		0.005198415	0.007892445	0.017329391	0.002716382	0.008041	-0.00456452	-0.00721933	-0.0062563	0.0090067	0.02650154	0.0240635	-0.00317393
Weekday returns
S&P 500		M	T	W	R	F
		0.000873526	0.000511974	0.000198215	0.000225441	0.000423
Russell 2000		M	T	W	R	F
		0.00078522	0.000415306	0.000444457	0.000570512	-0.0003
Monthly returns, enitre History
S&P 500		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.00034465	0.004193354	0.013063037	-0.00050614	0.006693	0.01581164	0.00718135	-0.0161839	0.00517672	0.00853357	0.0141442	0.011587135
Entire Monthly returns		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
		-0.02%	0.30%	1.31%	-0.05%	0.67%	1.58%	0.72%	-1.62%	0.52%	0.85%	1.41%	1.18%
Returns 1998-current		Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
S&P		-0.1602%	1.1053%	2.4942%	0.2709%	-0.5991%	1.5310%	-0.1681%	-1.2141%	2.2306%	2.0502%	0.8579%	0.3532%
Russell 2000		0.5198%	0.7892%	1.7329%	0.2716%	0.8041%	-0.4565%	-0.7219%	-0.6256%	0.9007%	2.6502%	2.4064%	-0.3174%
Weekday returns
S&P 500		M	T	W	R	F
		0.0879%	0.0506%	0.0193%	0.0229%	0.0434%
Russell 2000		M	T	W	R	F
		0.0792%	0.0417%	0.0437%	0.0577%	-0.0270%